Five years ago, on Sept. 11, 2001, terrorists crashed two airliners into the World Trade Center. Downtown smelled like Coke cans and hair on fire. It was televised live.

In New York City, 2,749 people were killed. About eight million remained. Since that day, the numbers have changed.

The population grew by more than 134,000 from 2000 to 2005, the city’s latest Planning Department calculations show. In that time, 645,416 babies were born and 304,773 people died. A half-million more people came from other countries than departed for them, and 800,000 more people left for the 50 states than came wide-eyed from them.

The meaning in the math is that today a great many New Yorkers lack firsthand knowledge of the city’s critical modern moment.

Five years on, New York is a city of newcomers and survivors. And between them runs a line. The line makes for no conflict, no discernible tension; it works a quieter breach..............................On the side of those who lived in New York, you can share a sense of trauma both layered and ill-defined.

“It’s like someone who has been in a war zone,” said William Stockbridge, 50, a finance executive who was working downtown during the attack. “It’s different.”.............................

Across the line, the new arrivals recognize that sense of ownership.

“I’ve been told that I just don’t get it and that I could never understand what it was like to be there in New York on Sept. 11,” said Laura Bassett, 27, who moved to the city from North Carolina after 2001. “I hate that five years later, people still debate which bystander is allowed to be more upset, the New Yorker or the American.”

The line emerges perhaps most powerfully around the fallen towers, 2.06 acres of concrete known as ground zero. Because of the line, the site is a paradox, an emotional contradiction, a mass grave and a tourist attraction.

Some people feel so strongly about the place they cannot agree on an arrangement for listing the names of the dead; others feel so strongly about the place that they make sure to visit between Radio City Music Hall and the Statue of Liberty. Between those emotional poles is a middle ground, and the line runs through its center.

“People who moved to New York, everyone wanted to go down and see it,” said Dede Minor, 51, a real estate broker who was in her office in Midtown on the day of the attack. “For New Yorkers, it was too real.”...............................

But across the line, that sense of energy is tempered by standards for comparison.

“I know people who have been here a year or two, and they find New York fantastic,” said Father Bernard, 67, a Roman Catholic monk who was born in Brooklyn and who goes by only that name. “They’re right, but they didn’t know the New York before.”

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Among those who have come to the city since 2001, the line dividing memories is undisputed.

“I had been there as a tourist to the World Trade Center, so I have memories,” said Marielle Solan, 22, a photographer who moved to the city from Delaware this year. “But obviously I can’t have any sense of what it was like. Every Sept, 11, you get a sense of fear and depression, but in terms of actual visceral reactions, I don’t really have that.”

The new arrivals have found a conspicuous void of shared memory.

“I’m amazed because it was such a big event, and people never mention it,” said Deenah Vollmer, 20, who moved to the city last year. “When you do mention it, everyone has these crazy intense stories.”

Across the line, many of those who lived in the city hold their memories close.

“The people I already knew know my stories from that day, so there’s no need to repeat them,” said Ms. Spielman, the graphic designer. “The new people I’ve met don’t ask me. It’s not something I bring up.”

But each year the calendar brings it up. Alexandria Lambert, 28, who works as an administrative assistant, sees the line run through the center of her office. Each year, a co-worker who witnessed the attack asks for the day off, and each year a boss who did not declines the request.

“His point of view is, ‘Don’t let it get you down,’ ” Ms. Lambert said, “but she just doesn’t want to be here.”

Until September 12, 2001, I never got why veterans were so reluctant to talk about their experiences. Now I understand.

I don't watch any documentaries or movies about 9/11. I don't watch the ceremonies. I cringe every cloudless, perfect day, a day New Yorkers call a 9/11 day.

I have written about 9/11 here because there wasn't much choice in it, but the last fucking thing I want to do is relive that day to satisify your curiousity. We who lived through that year, not day, it wasn't a fucking day, do not want to relieve it because you need some answer to your curiousity.

"Don't let it get you down?" That man deserved to be beaten to within an inch of his life.

Let me explain something: they buried the last firefighter from 9/11 last year. They found a bone or two with his DNA.

In the days after 9/11 there were pictures of the missing outside hospitals. Hundreds of faces. All dead, as it turned out.

Then the year of newspaper articles on a funeral. Seven days a week, for a year. And some girl wonders why she can't "share" in this? Share? There is nothing to share. There is grief, a sense of loss, and an understanding which you can never share. Any more than I can share what happened on Omaha Beach. I can understand, but it wasn't my friends dying there.

You want to understand 9/11, buy a fucking book and leave people to their memories.